1. Field of the Invention
The present invention relates to a piezoelectric sensor which senses a substance to be sensed in a sample fluid based on a change in natural frequency of a piezoelectric piece by making an adsorption layer formed on an electrode provided on the piezoelectric piece adsorb the substance to be sensed, and to a sensing instrument sensing a substance to be sensed by using the piezoelectric sensor.
2. Description of the Related Art
As an instrument for sensing a trace substance in a solution or gas, there has been known a sensing instrument which uses QCM (Quartz Crystal Microbalance) formed by a quartz resonator which is a piezoelectric resonator mainly formed by an AT-cut crystal piece as a quartz piece. A sensing instrument of this type senses a trace substance by making the quartz resonator included in a quartz oscillator circuit adsorb the trace substance and detecting a change in its oscillation frequency (resonant frequency). Examples of the trace substance are dioxin which is an environmental pollutant, a specific antigen in blood or serum, and so on, and the sensing instrument senses an extremely low concentration, for example, on ppb to ppt level, of these substances.
FIG. 10(a) shows an example of the structure of such a sensing instrument, in which 101, 102 denote a quartz resonator and an oscillator circuit forming a quartz oscillator circuit respectively. Further, on a subsequent stage of the oscillator circuit 102, a frequency detecting circuit 103 is connected to detect a change in oscillation frequency of the quartz resonator 101. One surface side of the quartz resonator 101 is in contact with an airtight space and the other surface side thereof is in contact with a measurement atmosphere to which a sample solution containing a substance to be sensed is supplied. An adsorption layer is formed on a front surface, which is the other surface side, to adsorb the substance to be sensed. At time t1 shown in FIG. 10(b), the sample solution is supplied to the quartz resonator 101, and when the substance to be sensed is adsorbed, the oscillation frequency of the quartz resonator 101 lowers due to a mass load effect. Based on the frequency change, the substance to be sensed in the sample solution is detected or the concentration thereof is measured.
However, the oscillation frequency of the quartz resonator 101 sometimes changes by factors other than the adsorption of the substance to be sensed. A possible factor may be the influence of the vibration given to the quartz resonator, for example, when a person walks in a room in which the sensing instrument is installed, but especially because the frequency of the quartz resonator changes depending on the temperature, there is a concern that the frequency may be influenced by a temperature change of a measurement environment due to an air conditioner, weather, and the like. When such a frequency change is caused by such a factor other than the adsorption of the substance to be sensed, measurement accuracy lowers, which as a result may possibly cause cases where, for example, the determination result becomes “equal to or lower than tolerable concentration” even though a toxic substance whose concentration is over a tolerable range is contained in a river, or the determination result becomes “present” even though a cancer marker is not present in blood, and the incorrect recognition causes a crucial situation.
Therefore, some sensing instrument has the structure shown in FIG. 11. 104A and 104B denote quartz resonators similar to the quartz resonator 101, but the quartz resonator 104A does not have the adsorption layer and is structured as a quartz resonator for reference not adsorbing a substance to be sensed. By changing a switch 106, oscillation outputs of the quartz resonators 104A, 104B are output to a frequency detecting circuit 103 in a time-division manner. Then, an arithmetic part 107 connected to the frequency detecting circuit 103 creates time-series data F1, F2 of the oscillation frequencies of the quartz resonators 104A, 104B detected by the detecting circuit 103, and further calculates a difference between F1 and F2 to create time-series data F1-F2. That is, since a frequency change ascribable to a temperature change is reflected in the time-series data F1 and a frequency change ascribable to the adsorption of the substance to be sensed and the temperature change is reflected in the time-series data F2, the time-series data F1-F2 corresponding to a difference therebetween represent the frequency change in which the influence of the temperature change is cancelled and which is thus caused only by the adsorption of the substance to be sensed. Therefore, the use of the time-series data F1-F2 as a basis of the detection of the substance to be sensed enables improved measurement accuracy.
Incidentally, in order to more improve the measurement accuracy by realizing higher frequency stability against temperature, it has been considered to structure a quartz oscillator circuit by using a quartz resonator in which two vibration areas are set on one quartz piece and excitation electrodes are formed on the quartz piece so as to correspond to the respective vibration areas, and to detect the influences of both the adsorption of the substance to be sensed and the temperature change in a vibration part including one of the vibration areas and detect only the influence of the temperature change in a vibration part including the other vibration area, as in the aforesaid sensing instrument in FIG. 11. That is, in this example, the single quartz piece functions as two quartz resonators and the vibration areas are formed on the common quartz piece, and therefore, it is possible to obtain frequency-temperature characteristics in which variations of the oscillation frequencies output from the respective vibration areas when an ambient temperature changes are substantially equal. Then, based on the difference between the time-series data F1, F2 which are obtained from the respective vibration areas as in the aforesaid sensing instrument in FIG. 11, a substance to be sensed is detected. Patent documents 1, 2 describe such a quartz oscillator circuit.
FIG. 12 shows a quartz oscillator circuit 110 including a quartz piece 111 as structured above. 112, 113 in FIG. 12 denote excitation electrodes corresponding to a first vibration area 114 and a second vibration area 115 of the quartz piece 111 respectively. 116 in FIG. 12 denotes a common excitation electrode commonly used for the vibration areas 114, 115. 117 denotes the aforesaid quartz resonator, which includes: the first vibration area 114 including the quartz piece 111 and the electrodes 112, 116; and the second vibration area 115 including the quartz piece 111 and the electrodes 113, 116. The first vibration area 114 and the second vibration area 115 are structured to vibrate independently of each other, the former being vibrated by the electrodes 112, 116 and the latter being vibrated by the electrodes 113, 116.
Since, on a microscopic level, the first vibration area 114 and the second vibration area 115 are different in thickness and the electrodes 112, 113 are different in thickness, frequencies of the first vibration area 114 and the second vibration area 115 are slightly different, the difference therebetween being, for example, on a several ten kHz order in a case of a 9 MHz quartz resonator. If their frequencies are thus extremely close to each other, they are attracted to each other, resulting in frequency signals with low Q values. Therefore, the electrodes 112, 113 are made slightly different in thickness so that the frequencies differ slightly. 123, 124 denote a set of two Colpitts oscillator circuits serially connected to the quartz resonator 117 in order to take out the oscillation frequencies by using the vibration areas 114, 115.
FIG. 13 is a side view of the quartz resonator 117, in which 119 denotes a board, 122 denotes an airtight space in contact with a rear surface of the quartz resonator 117, and 120 denotes an adsorption layer adsorbing a substance to be sensed contained in a sample solution 118 supplied to a front surface of the quartz resonator 117.
Incidentally, in the sensing instrument including the quartz oscillator circuit 110, a frequency difference of one of the vibration areas is corrected by the frequency difference of the other vibration area to remove the influence of the temperature characteristic, whereby only a variation of the frequency ascribable to the adsorption of the substance to be sensed is extracted, as described above. Therefore, it is necessary to vibrate the two vibration areas 114, 115 independently of each other. Therefore, when the quartz resonator 117 is structured as having the two vibration areas 114, 115 on the single quartz piece 111 as described above, it is necessary to reduce the mutual influence of the vibration between the vibration areas 114, 115. However, as shown in FIG. 13, when the quartz resonator 117 in contact with the sample solution 118 is seen from an electrical point of view, the excitation electrode 112 of the vibration area 114 and the excitation electrode 113 of the other vibration area 115 are connected to each other via a low-resistance component 121 shown by the dotted line in FIG. 13, due to the impedance of the sample solution 118 itself. FIG. 14 is a Smith chart showing the measurement result of the resistance component 121, and as is seen in the Smith chart, when the quartz oscillator circuit 110 uses a frequency of, for example, 9.125 MHz, the impedance is about 100 Ω.
The presence of such a resistance component 121 affects the oscillations of the oscillator circuits 123, 124, and it becomes difficult to maintain the oscillations of the vibration areas 114, 115, leading to a failure in the measurement, or an unnecessary component generated by the influence that the vibration of one of the vibration areas gives to the vibration of the other vibration area is output in the frequency signal taken out from each of the vibration areas, leading to a difficulty in highly reliable measurement.
[Patent Document 1]
Japanese Patent Application Laid-open No. 2006-33195: claim 1, paragraph 0012 to paragraph 0014, paragraph 0018 to paragraph 0019, FIG. 1, FIG. 4
[Patent Document 2]
Japanese Patent Application Laid-open No. 2006-3144, paragraph 0015, paragraph 0023, FIG. 2